Xylitol was first reported by a chemist Emil Fisher in 1981 and is a five-carbon sugar alcohol that has been used as a sweetener since 1960. Xylitol is found in small quantities in natural plants such as fruits, vegetables, and mushrooms, and is an intermediate of mammalian carbohydrate metabolism. Furthermore, because of its sweetness equivalent to sucrose and its negative heat of dissolution, xylitol imparts a cool and refreshing sensation in the oral cavity. Therefore, xylitol is used as a material for sugar-free confectionery products. In particular, since it does not require insulin to be metabolized after its intake, xylitol can be used as a sugar substitute for diabetic patients.
Xylitol can prevent cavities by inhibiting the growth of Streptococcus mutans which is responsible for tooth decay, and thus, is used as a material of toothpaste. Furthermore, xylitol does not participate in the Maillard reaction, and is a monosaccharide and thus cannot undergo inversion unlike sucrose. Therefore, xylitol has no risk of degeneration even when used at an acidic environment. Still furthermore, xylitol has a low boiling point of 95° C., and thus, can reach its boiling point with no denaturalization. Therefore, when used as a sugar-coating, xylitol is not particularly required to be dissolved in water.
Xylitol is produced on an industrial scale by chemical reduction of a hydrolysate of hemicellulose taken from plants such as white birch trees, corn cobs, etc. or by biological conversion of the hydrolysate to xylitol using a microbial strain. With respect to the chemical production method, however, separation and purification of xylose or xylitol from other hydrolysates derived from hemicellulose fractions are difficult and the yield of xylose or xylitol is as low as 50-60%. Furthermore, there arise problems such as a risk of high-temperature and high-pressure reaction and waste disposal due to use of alkali. To solve these problems, industrial-scale production of xylitol using a microbial strain has been reported. Xylitol production using a microbial strain is cost-effective, and enables selective conversion of xylose to xylitol, thereby facilitating separation and purification of xylitol. However, there are disadvantages in that xylitol productivity is as low as 2.0-3.0 g/l-h and a microbial strain can be used only once. For this reason, once culturing is finished, conventional xylitol production techniques involve preparative procedures for re-culturing, such as washing and sterilization, thereby leading to an increase of production cost.
In xylitol production using a strain of the genus Candida, xylose or hemicellulose hydrolysate comprised mainly of xylose is used as a carbon source. As a nitrogen source, there is used a complex nitrogen source containing various vitamins such as yeast extract, malt extract, soybean meal, etc. This is because strains of the genus Candida are relatively complex auxotrophs, thereby scarcely producing xylitol in a chemical synthetic medium. In this respect, use of an expensive complex medium becomes a contributing factor to the increased cost of xylitol.
Therefore, to solve the problems of the above-described xylitol production using a microbial strain, the present inventors developed a process for producing xylitol from a culture of a microbial strain cultured in a chemically defined medium. According to this process, the microbial strain is concentrated and recycled after isolated from the culture. This xylitol production process can be applied to an automated continuous culture system, thereby leading to reduction of a production cost due to a simplified culture procedure and continuous production of xylitol. The present inventors thus completed the present invention.